Mannich base reactions are well-known. Mannich base compounds are products based on the reaction of an aldehyde, generally formaldehyde, a phenolic compound and an amine. Various forms of phenolic compounds, amines and aldehydes have been proposed. Mannich base products are known to be suitable for curing epoxy resins.
Phenalkamine curing agents are a class of Mannich bases obtained by reacting a cardanol-containing extract derived from cashew nutshell liquid, an aldehyde compound, such as formaldehyde, and an amine. Commercially available phenalkamines use ethylenediamine and diethyltriamine as the amine. Phenalkamines are good epoxy resin hardeners for room temperature or low temperature curing applications. Phenalkamines, however suffer from the disadvantage that the cured epoxy articles are very dark in color (Gardner Color Index greater than 14), blushing and low mechanical properties. Due to the dark coloring of the cured coatings, epoxy resin compositions containing phenalkamine curing agents have been limited to use as primers and generally cannot be used as top coat material.
The extract from cashew nutshell liquid primarily contains a mixture of cardanol (I), cardol (II), ##STR2##
where n depends upon whether the side chain is saturated or unsaturated (n=0 for saturated, n=2 for monoene, n=4 for diene, and n=6 for triene), and related compounds of varying degrees of saturation. Numerous methods have been developed to purify the extract from cashew nutshell liquid with the stated goal of obtaining an isolated solution of cardanol. Typically, a purified extract solution will contain primarily cardanol with a minor amount of cardol.